If parasites are causing this problem, you should be able to see them on the chicken by looking at the skin around the vent for tiny beasties on the move. If you see them, the next step is going to the store and buying some poultry permethrin dusting powder and using it on the chickens.
If the feather loss and low energy is being caused by molt, that's also easy enough to verify. Look on the back of the neck, back in front of the tail and below the vent for uniform pin feathers erupting through the skin. If you see them, it's normal for molt for a hen to be lethargic on lose her appetite.
The next obvious cause of a chicken feeling unwell is a crop disorder. You can verify this easily by checking the crop early in the morning before feeding. Full crop in the morning is a sign of a sick crop.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/
The next in order would be reproductive issues. This requires a lot more than just a quick look. Age of the hen, when she last laid an egg, the quality of the egg, poop condition and color, and how long she's been acting off, all need to be considered in order to arrive at this diagnosis.
From here on, it's going to be strictly guess work on our part out here in the peanut gallery. A bacterial or viral infection may be what's causing the chicken to act sick. You need to give us a lot more background in order for us to give advice. if she's eating at all, whether she can stand on her own, whether her eyes are closed most of the time all are important clues. You might be just as well off seeking an appointment with a vet to do the necessary exam required to judge the health of the chicken. It could end up being a respiratory illness or simple a sore on a foot that's infected.
So, go back and rule out the first several possible issues, maybe take some photos of your hen and post them, and tell us what you've found out by examining your hen. We'll take it from there and see what we can come up with.
